FIG. 13 shows an equivalent circuit of a pixel circuit of a typical active matrix type liquid crystal display device. In addition, FIG. 14 shows a circuit arrangement example of the active matrix type liquid crystal display device having m×n pixels. As shown in FIG. 14, a switch element including a thin film transistor (TFT) is provided at each of intersecting points of m source lines (data signal lines) and n scanning lines (scanning signal lines), and as shown in FIG. 13, a liquid crystal element LC and a retentive capacity Cs are connected in parallel through the TFT. The liquid crystal element LC has a laminated structure in which a liquid crystal layer is provided between a pixel electrode and an opposite electrode (common electrode). In addition, FIG. 14 only shows, in a simplified manner, the TFT and the pixel electrode (black rectangular part) in the pixel circuit. The retentive capacity Cs has one end connected to the pixel electrode, and another end connected to a capacity line LCs, and stabilizes a voltage of pixel data held in the pixel electrode. The retentive capacity Cs has effects of preventing a fluctuation of a voltage of the pixel data held in the pixel electrode due to a leak current of the TFT, a fluctuation of electric capacity of the liquid crystal element LC between a black display and a white display due to dielectric constant anisotropy of liquid crystal molecules, and a voltage fluctuation generated due to parasitic capacity between the pixel electrode and a surrounding wiring. By sequentially controlling a voltage of the scanning line, the TFT connected to the scanning line is turned on, and the voltage of the pixel data supplied to the source line is written in the corresponding pixel electrode with respect to each scanning line.
In a normal display by way of a full-color display, even when display contents are still images, the same display contents are repeatedly written in the same pixel with respect to each frame, with the polarity of the voltage applied to the liquid crystal element LC being reversed every time, so that the voltage of the pixel data held in the pixel electrode is updated, the voltage fluctuation of the pixel data is suppressed to a minimum, and a high-quality display of the still image is maintained.
Power consumption to drive the liquid crystal display device is mainly dominated by power consumption to drive a source line by a source driver, and roughly expressed by a relational expression shown in the following formula 1. In the formula 1, P represents power consumption, f represents a refreshing rate (the number of times of refreshing actions for one frame per unit time), C represents load capacity driven by the source driver, V represents a drive voltage of the source driver, n represents the number of the scanning lines, and m represents the number of the source lines. It is to be noted that the refreshing action mean an action to clear a fluctuation generated in the voltage (absolute value) applied to the liquid crystal element LC and corresponding to the pixel data by rewriting the pixel data, and to return the voltage to the original voltage state corresponding to the pixel data.P∝f·C·V2·n·m  Formula 1
Meanwhile, in the case where a still image is constantly displayed, since the display contents are still images, it is not always necessary to update the voltage of the pixel data with respect to each frame. Therefore, in order to further reduce the power consumption of the liquid crystal display device, a refreshing frequency is reduced at the time of this constant display. However, when the refreshing frequency is reduced, the pixel data voltage held in the pixel electrode fluctuates due to a leak current of the TFT. In addition, since an average potential is also reduced for each frame period, this voltage fluctuation leads to a fluctuation of display brightness (transmittance of the liquid crystal) in each pixel, which is recognized as a flicker. In addition, this may cause reduction in display quality such that sufficient contrast cannot be obtained.
Here, as a method for solving a problem of reduction in display quality due to the reduction in the refreshing frequency at the time of the constant display of the still image, for example, configurations are disclosed in the following patent documents 1 and 2. According to the configurations disclosed in the patent documents 1 and 2, the switch element of the pixel circuit shown in FIG. 13 is constituted by a series circuit including two TFTs (transistors T1 and t2), and its middle node N2 is driven so as to have the same potential as that of a pixel electrode N1 with a unity gain buffer amplifier 50, to prevent a voltage from being applied between a source and a drain of the TFT (T2) arranged on the side of the pixel electrode, so that a leak current of this TFT is considerably suppressed, and the problem of reduction in display quality can be solved (refer to FIGS. 15 and 16).
This is a method for a solution provided based on the fact that the leak current of the TFT considerably increases in association with an increase of a bias voltage between the source and the drain. As shown in FIGS. 15 and 16, according to the configurations described in the patent documents 1 and 2, as for the TFT (T1) connected to a source line SL, the bias voltage between the source and the drain increases and the leak current of the TFT could increase, but since the leak current is compensated by the buffer amplifier 50, it does not affect a pixel data voltage held in the pixel electrode N1. Thus, when the buffer amplifier 50 is provided, the problem of the reduction in display quality due to the reduction of the refreshing frequency can be solved, and power consumption can be reduced due to the reduction of the refreshing frequency. In addition, the configurations described in the patent documents 1 and 2 can be applied to two or more different voltage states as the pixel data voltages held in the pixel electrode, so that a multi-gradation constant display can be implemented with high display quality and low power consumption.